Among Democrats, Kerry's replacement in the Senate, Ed Markey, D-Mass., voted "present" after expressing misgivings.
In his comments in Sweden, the president sought to shift the onus for responding to Assad to Congress and the world at large.
"I didn't set a red line. The world set a red line" with a treaty banning the use of chemical weapons. He added that "Congress set a red line" when it passed legislation a decade ago demanding Syria stop production of weapons of mass destruction.
His comments drew a response of disbelief from one Republican back home.
"He needs to go back and read his quote," Sen. Saxby Chambliss of Georgia said, referring to a statement the president made slightly more than a year ago.
On Aug. 20, 2012, Obama said: "We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. . . . That would change my calculus" about military action.
• In Syria, al-Qaida-linked rebels were said to have launched an assault on a government-held Christian mountain village in the densely populated western part of the country, and there was new fighting near Damascus, as well.
• In Rome, Pope Francis underscored Vatican opposition to military strikes against Syria, urging Catholics and non-Catholics alike to take part in a day of fasting and prayer for peace on Saturday. He has called for a negotiated settlement in Syria and also has condemned the use of chemical weapons.
• In France, Prime Minister Jean-Marc Ayrault told Parliament that failure to take action would allow Assad to launch more chemical attacks.
In Washington, Kerry said Assad had used chemical weapons 11 times, including once last spring. At that time, he said, Obama did not have a "compelling" enough case to push for a U.S. military response.
As for the most recent reported chemical weapons attack, Kerry declared that "only the most willful desire to avoid reality can assert that this did not occur as described or that the regime did not do it. It did happen -- and the Assad regime did do it."
Few, if any, members of Congress dispute the administration's claim that Assad was responsible for the reported attack, and lawmakers in both parties appear far more focused on determining how they should respond.
Gaveling the House committee hearing to order, Rep. Ed Royce, R-Calif., said that while it would be important to deter the use of chemical weapons by Assad and others, there remains many unanswered questions, including what the United States would do if Assad retaliates.
"The administration's Syria policy doesn't build confidence," he said.